Build with serde support on CI

[SimonSapin]

Do something similar to #85 (comment) for every optional features.

[mylainos]

So we should add something like this to .travis.yml at line 13 ?

  - ([ $TRAVIS_RUST_VERSION != nightly ] || cargo test --features serde-serialization)

[SimonSapin]

Indeed! Thanks for reminding me.